Increasingly, applications require that high aspect ratio trenches and/or vias be filled with powders or paste materials as a step in the fabrication of z-direction electrical circuits. One such application is the formation of z-direction line circuitry in ceramic, glass, silicon, or organic substrates. Such vias have high aspect ratios which make them difficult to fill. Particularly when they are so called “blind” via or closed at one end and open on the other. For instance they are sometimes only tens of microns in diameter, while being hundreds of microns in depth. Traditional methods for filling pastes, powders, suspensions and viscous fluids into such vias are not adequate.
One traditional method for filling high aspect ratio vias with metal pastes requires the creation of a partial vacuum, typically in the milli torr range, followed by the application of metal paste onto the substrate surface and squeezing the paste over and into the via. If the viscosity is low enough, the paste will push into the via. There are however two significant drawbacks to this method.
A first drawback, illustrated in FIG. 1, relates to the pocket of gas that is trapped in the via. FIG. 1 shows a substrate 10 having vias 12 filled with copper coated tungsten powder which has been sintered post-filling. The copper sinter manifests significant voiding 14 that arises because the vacuum is generally poor (i.e. of fairly high pressure in the millitorr range). When the paste is screened or squeezed into the via, the via becomes covered with paste and a quantity of gas in the volume of the via is trapped. Reducing fill chamber vacuum may result in undesirable paste drying due to solvent removal. Thus a compromise between fill chamber vacuum and retained void must be made. As the paste is subsequently packed, the gas is compressed and pushed to the via bottom. This gas bubble cannot escape through the very high viscosity paste and results in a permanent void. This void can be large enough to preclude reliable electrical interconnection through that via, and also constitutes a structural defect. FIG. 1 also shows that the vias are open to the substrate via-surface.
A second deficiency relates to the composition of metal pastes, which, of necessity, consists of a mixture metal particles and a liquid medium. After application, the paste is dried and the liquid medium is removed leaving a pure metal packing ready to be sintered to a solid. The removal of the liquid medium causes shrinkage and often requires a second (or third or more) application of paste to achieve reasonably complete filling. Several iterative evacuation, filling, drying, and sintering steps may be required to properly fill vias and trenches. However, the multiple steps are very expensive increasing turn-around-time.
A need therefore exists for a single-step method to fill high-aspect ratio trenches and vias in a manner that avoids or minimizes void formation.
Other objects and advantages will become apparent from the following disclosure.